Dynamic Charge Equilibration Force Field for Modeling Zeolites

Jan Sefcik, Ersan Demiralp, Tahir Cagin and William A. Goddard III

        Charge Equilibration (QEq) method (Rappe and Goddard, 1991) has been previously successfully used to calculate geometry dependent atomic charges in molecules and solids. The purpose of assigning sensible atomic charges is to reproduce realistic electric fields for modeling of catalysis and adsorption in pores of zeolites. In Dynamic Charge Equilibration (DQEq) method we calculate electrostatic energy and its derivatives with respect to atomic coordinates self-consistently based on the QEq approach so that we obtain corresponding forces and hessians for molecular mechanics, molecular dynamics and vibrational analysis. Force fields coupled with DQEq allow charges to readjust as geometry changes in classical simulations. DQEq electrostatics is then the base on the top of which one can build force fields of his choice. We developed a widely transferable, simple non-bond force field with DQEq electrostatics in order to describe SiO2 and AlPO4 phases, including zeolite analogues. In addition to DQEq electrostatics, we used only two-body interactions modeled by Morse terms. This keeps the force field as simple as possible, while being able to capture quantitatively desired properties of materials of interest. We parameterized the two-body terms by fitting structures, elastic properties and thermochemistry of few selected phases and then transferred the force field to all other known polymorphs. The force field was tested on all well characterized crystalline phases of SiO2, and AlPO4. All available experimental data on geometry and relative energetics of these phases were reproduced very well. Since DQEq does not require a priori assignment of atomic charges, it is very suitable for simulation of real world zeolite systems with defects and extra framework molecules.

        This research was funded by the BP-Amoco.

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